Why are there differences between FAA, UK CAA, Eurocontrol and Icao?
Because everyone beleives their views are better, or they have a bigger stick .
For instance the USA often refuses ICAO when it does not suit them :
a quote from a former FAA top executive during a famous meting on " line up and wait" : "we have 600.000 pilots and 20.000 controllers in the USA following good sound FAA procedures, you want me to tell them to change their ways because a UN organisation is saying so ? "
(for the young ones here, "UN" was a dirty word then in the USA, opposing Iraq intervention etc..)